The Greatest Actress of all time is Marilyn (Monroe). I admit that, despite idolizing Marilyn since childhood, as far back as forever, my familiarity with her actual performances in film is limited.
That being said, I think she was probably always typecast - as they call it - as the blonde sex symbol/leading lady of love and beauty and desirability; but, like the Scarecrow character in the Oz MGM film: Brainless, an airheaded type of female. Meaning: I do not think she was particularly seen [as can be said was the view of most women by the societal male view during her era/time], nor probably cast either, as any intellectual roles/smarty pants-type, etc.... Hopefully, you get my gist.
In reality, she was rarely seen without something in her hand that she was reading intently - and it was not just her scripts to memorize lines; but, also magazines like Time or National Geographic, or a book - this is according to the documented observation of those who were close and personally knew the real person. Therefore: if you combine this obvious ability to suspend the reality of her true self to become the ditzy character in her work (as is the goal of all good acting – suspension of the audience’s disbelief), add that to the fact that of all her photos that exist, and there are countless images out there of the legendary hottie; but, none of which are showing anything other than impeccable style, flawless-seeming beauty within and in appearance outside, nor even caught off guard in any shameful act, nor unprepared and surprised with an awkwardness in the expression on her face – nothing but gorgeous pictures exist of the woman. This fact is amazing to me, since - based on the sheer size/# of pictures within the whole Marilyn Monroe Image Catalogue, as in every photograph EVER taken of her that exists [ALL of them!] - are so unimaginably numerous and were taken over a fairly short time period that she had her fame, as well, increasing the awe-factor of her consistent desired image that was maintained, making it look like never did she have a private moment spent entirely alone (except for the autopsy photos – they do not qualify as being anything described here). These thigs, when all combined and considered, are enough to be evidence of her worth that of deeming her the "greatest actress" of all time.
Or, if acting ability alone is the only factor to consider, then NEVER MIND. :) K.